1. Field of the Invention
The present invention relates to a cylinder head and motor block casting, including an aluminum alloy having the following composition: Si 6.80-7.20, Fe 0.35-0.45, Cu 0.30-0.40, Mn 0.25-0.30, Mg 0.35-0.45, Ni 0.45-0.55 Zn 0.10-0.15, Ti 0.11-0.15 with the remainder being aluminum as well as unavoidable impurities with a maximum content of 0.05 each, but not more than a maximum of 0.15 impurities in all.
2. Description of the Related Art
The properties of aluminum depend on a number of factors whereby added or accidentally present admixtures and impurities of other elements play an important part. The main alloying elements are copper (Cu), silicon (Si), magnesium (Mg), zinc (Zn) and manganese (Mn).
It often happens that the following impurities or additions are contained in small quantities: iron (Fe), chromium (Cr) and titanium (Ti). The following additions are used for special alloys: nickel (Ni), cobalt (Co), silver (Ag), lithium (Li), vanadium (V), zirconium (Zr), tin (Sn), lead (Pb), cadmium (Cd) and bismuth (Bi).
All alloy constituents are completely solvable in liquid aluminum at a high enough temperature. The solubility in the solid state with formation of solid solutions is limited for all elements; there is no alloy system comprising aluminum which shows a uninterrupted solid solution sequence. The unsolved parts form their own phases, so-called heterogeneous constituents, in the alloy micro structure. They are often hard and brittle crystals made up of one element alone (e.g. Si, Zn, Sn, Pb, Cd, Bi) or consisting of intermetallic compounds comprising aluminum (such as Al2Cu, Al8Mg5, Al6Mn, Al3Fe, Al7Cr, Al3Ni, AlLi). Alloys having two or more constituents contain in addition to these intermetallic compounds, yet other intermetallic compounds consisting of the additions (e.g. Mg2Si, MgZn2), ternary phases (e.g. Al8Fe2Si, Al2Mg3Zn3, Al2CuMg) and phases comprising even more constituents. The formation of solid solutions and the formation of the heterogeneous micro structure constituents (their amount, size, form and distribution) determine the physical, chemical and technological properties of an alloy. Due to the fact that the diffusion rate decreases with temperature it is possible, after a rapid cooling from higher temperatures, that Al-solid solutions may contain higher levels of solved elements than would be possible in equilibrium at room temperature. In such over saturated solid solutions precipitation processes may occur at room temperature or at moderately raised temperatures (partly with formation of metastable phases), these may be of great influence on the properties. Elements which diffuse slowly such as Mn can be over saturated far beyond the maximum equilibrium solubility by rapid solidification from the melt. This over saturation may be remedied by annealing at high temperatures. The additions are then precipitated in a finely dispersed manner. Often this annealing process (full annealing) is used for compensating micro segregation.
Below some important binary and ternary systems are described with short explanations:
Aluminum-copper
In the range of 0 to approximately 53% Cu there is a simple eutectic sub-system with a eutetic at 33.2% Cu and 547xc2x0 C. The maximum solubility at the eutectic temperature in the alpha solid solution is 5.7%. The solubility decreases with falling temperature and is only 0.45% at 300xc2x0 C. Unsolved copper is present in the form of Al2Cu in the state of equilibrium. Metastable transition phases may be formed at medium temperatures by precipitation from the oversaturated solid solution.
Aluminum-silicon
This system is purely eutectic having a eutetic at 12.5% Si and 577xc2x0 C. At this temperature 1.65% Si are solvable in the alpha solid solution. At 300xc2x0 C. only 0.07% are solvable. The crystallisation of eutectic silicon may be influenced by small amounts of additions (e.g. of sodium or strontium). In this case an overcooling and shift of concentration of the eutectic point occur in dependence on the solidification rate.
Aluminum-magnesium
The subarea between 0 and approx. 36% Mg is eutectic. The eutetic is at approximately 34% Mg and 450xc2x0 C. At this temperature the (maximum) solubility is 17.4% Mg. At 300xc2x0 C. 6.6% and at 100xc2x0 C. about 2.0% Mg are solvable in the alpha solid solution. In most cases unsolved Mg is present in the microstructure in the form of the xcex2-phase (Al8Mg5).
Aluminum-zinc
The alloys form a eutectic system having a high-level zinc eutetic at 94.5% Zn and 382xc2x0 C. In the area high in aluminium, which is of interest here, 31.6% Zn are solvable at 275xc2x0 C. in the solid solution. The solubility is very much dependent on the temperature and falls to 14.5% at 200xc2x0 C. and to 3.0% at 100xc2x0 C.
The systems of aluminum-manganese, aluminum-iron and aluminum-nickel show a eutetic at a low concentration. The melting point is only very slightly lowered. The solubility in the solid state is low except that of manganese.
From the journal AFS Transactions, Volume 61, 1998, pages 225 to 231, it has been known to optimize aluminum-silicon cast alloys for cylinder heads by adding copper to them. In this case the thermal strength of an AlSi7Mg-alloy, to which 0.5 to 1% copper had been added, increased significantly whereby simultaneously the creep resistance also improved. The improvement of the mechanical properties, however, is accompanied by a deterioration of ductility and a reduced corrosion resistance.
After having manufactured the cylinder head and motor block castings in a casting process it is often necessary to carry out machining operations on them. In certain alloys problems occur as a result of too little hardness because the surfaces of the castings become very soft so that fine scoring or smudging may occur.
Furthermore, such alloys must have a high thermal conductivity so that the castings are suitable for use in motors. The piston alloys with 12% Si which have been examined by way of comparison do not meet the requirements, nor does the normally used AlSi9Cu3.
Therefore, an object of the present invention is to provide an alloy suitable for use in cylinder head and motor block castings, having a high thermal conductivity and an appropriate crystalline structure, high thermal strength, good creep resistance as well as sufficient ductility and, at the same time, having low vulnerability to corrosion and being easily machinable.
According to the present invention this object is accomplished by the features in the claims.
The research of the inventors has shown that cylinder head and motor block castings consisting of an aluminum alloy comprising the following composition:
Si 6.80-7.20
Fe 0.35-0.45
Cu 0.30-0.40
Mn 0.25-0.30
Mg 0.35-0.45
Ni 0.45-0.55
Zn 0.10-0.15
Ti 0.11-0.15
remainder aluminum as well as unavoidable impurities with a maximum content of 0.05 each, but not more than a maximum of 0.15 impurities in all, exhibits an especially high creep resistance and thermal strength, if phases in the amounts of 1 to 3 vol. % of the aluminum-nickel type, aluminum copper type, aluminum-manganese type, aluminum-iron type and mixed phases of the aforementioned types are contained and if, in particular, a ratio of Ni:Mg:Cu=5:4:3.5 is observed. The thermal conductivity and ductility of a cylinder head and motor block casting are improved by a crystalline structure consisting of an alpha aluminum matrix structure having 40 to 55 vol. % and by observing a Mn/Fe-ratio of at least 0.781. If the aluminum alloy elements are contained in the following ratios
Si:Fe:Cu=7:0.4:0.35
Ni:Mg:Cu=5:4:3.5
the cylinder head and motor block casting according to the present invention shows very good corrosion properties. It was found that cylinder head and motor block castings are easier to machine and have an improved hardness when they are produced in the following way:
An aluminum alloy is filled into a casting mold at a temperature of 720xc2x0 to 740xc2x0 C., then the aluminum alloy is subjected to cooling at a cooling rate of 0.1-10 K sxe2x88x921 and after cooling to room temperature a thermal treatment is carried out consisting of a solution heat treatment at 530xc2x0 C. for 5 hours, chilling in water at 80xc2x0 C. and artificial ageing at a temperature of 160 to 200xc2x0 C. for 6 hours.